1. Field of the Invention
The invention relates to limiting switches comprising, in a case: at least one mobile lever subjected to the action of a resilient member which applies one against the other contact pieces placed at the respective ends of the lever arm carrying the mobile contact and of a contact support substantially parallel to said arm; a thin insulating screen placed between said lever arm and the support so that its edge is inserted between the contact pieces, while maintaining the insulation between the lever and the support when these latter move away from each other following short circuit currents flowing in the switch.
2. Description of the Prior Art
Such switches, which are advantageously used for protecting more especially electricity consuming appliances of every kind, for example in conjunction with circuit breakers, may be illustrated by the French Patent Application No. 81 15573 filed on the 07.08 81 by the applicant for: "An automatic short circuit current limiting device". In the switch forming the subject of said application, measures are taken so that the arc appearing between the contact pieces situated in the vicinity of the ends or horns of two levers which form the supports for the contacts when these latter move away from each other under the effect of the electrodynamic forces which appear therebetween, is drawn out radially with respect to the axes of said levers, not only by the effect of the current loop formed by this arc, but also by the rapid movement of the edge of the screen, which extends the arc so as to lead it into a region between two extinction chambers provided with fractionizing fins in which this arc will be extinguished.
A study of the performance of a known switch of this type, namely: its low cut off time, the peak current which it limits to reduced values and the low energy dissipated, has shown that the technique used was particularly advantageous in a range of short circuit currents of intensities between low and medium values for, under these conditions the effects of lengthening and cooling the arc caused by the screen are preponderant. On the other hand, if it is desired to keep these performances in the range of higher intensities, with the known technique, it has been discovered that the speed of separation of the contacts and the natural swelling of the arc caused by the electrodynamic forces are such that the screen has not time to produce the desired effects; it has more especially been discovered, when the break is operated at these high intensity values, that relatively high arc energy is released at the time of breaking.